[PATCH v4 13/16] rust: drm: add AsRef<ParentDevice<Bound>> for Device<Registered>

[Danilo Krummrich <dakr@kernel.org>]

Implement AsRef<T::ParentDevice<Bound>> for Device<T, Registered>,
providing access to the bound parent bus device for registered DRM
devices.

Since a Device<T, Registered> guarantees that the parent bus device is
bound, the conversion to T::ParentDevice<Bound> is safe.

Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
 rust/kernel/drm/device.rs | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/rust/kernel/drm/device.rs b/rust/kernel/drm/device.rs
index 45bacd7b227f..8f63276c9b62 100644
--- a/rust/kernel/drm/device.rs
+++ b/rust/kernel/drm/device.rs
@@ -462,6 +462,19 @@ fn as_ref(&self) -> &T::ParentDevice<device::Normal> {
     }
 }
 
+impl<T: drm::Driver> AsRef<T::ParentDevice<device::Bound>> for Device<T, Registered> {
+    fn as_ref(&self) -> &T::ParentDevice<device::Bound> {
+        let dev = (**self).as_ref().as_ref();
+
+        // SAFETY: A `Device<T, Registered>` guarantees that the parent device is bound.
+        let dev = unsafe { dev.as_bound() };
+
+        // SAFETY: By the type invariant of `Device`, the parent device is embedded in
+        // `T::ParentDevice`.
+        unsafe { device::AsBusDevice::from_device(dev) }
+    }
+}
+
 // SAFETY: A `drm::Device` can be released from any thread.
 unsafe impl<T: drm::Driver, C: DeviceContext> Send for Device<T, C> {}
 
-- 
2.54.0